Flag setting in the Related Items Pane
 

There is a particular effect in the Related Items Pane. Suppose there are more items displayed. Item_A is selected (highlighted) and has any flag value (including None). With Ctrl+Arrow up/down, the focus rectangle is moved to Item_B. Attempt to change the flag value of Item_B: Item menu / Flag / select the same value as at Item_A. Not working. All other flag values are working.

Right-click the item or use the context menu key.

Thank You. Right click works right, context menu key works same manner as the mentioned Item menu.